It is known to manufacture containers of thermoplastic material by forming preforms, for example by blow molding or by stretch blow molding.
The preforms are in general manufactured by injection molding. A preform has a body that is intended to be stretched to be shaped into the final container during the forming operation. The preform also has a neck that is injected into its final shape. It is therefore important to protect the neck throughout the process for transforming the preform into the final container.
To carry out the forming operation, it is necessary to heat in advance the bodies of the cold preforms at a glass transition temperature so as to make them malleable. The facilities for mass-producing containers for this purpose comprise furnaces for heating preforms.
A heating furnace in general has the shape of a tunnel in which at least one of the walls comprises heating means. The preforms move along the tunnel in front of the heating means while turning round and round so that their bodies are heated in a uniform manner to a temperature that is suitable for the forming operation.
For this purpose, the furnace comprises a device for transporting preforms that in general comes in the form of a closed chain, each link of which forms a movable support that is provided with an associated device for gripping a preform.
Each transporting device comprises an end mandrel that is formed by a head in which the jaws are mounted to slide radially, with the jaws being returned to an expanded position by an elastic ring that is inserted between the jaws and the nose. The nose that is equipped with jaws thus forms a radial expansion mandrel that is forced inside the neck of the preform.
The nose is engaged with the neck of the preform during an insertion operation that is sometimes called “covering.”
At the outlet of the furnace, the preforms are transferred to a forming station, by blow molding or by liquid injection, by means of a transfer wheel. During the passage of preforms from the furnace to the transfer wheel, the mandrels are extracted from the neck of the preforms so that the preforms are ejected from the transporting device during an ejection operation, sometimes called “uncovering.” The thus ejected preforms are received in suitable slots of the transfer wheel.
The machines for mass-producing containers should allow preforms to move at high speed into the furnace. This involves being able to carry out the operations for inserting preforms and the operations for ejecting preforms at a fast enough rate not to slow down the movement of the preforms into the furnace.
In the known gripping devices, the jaws are free to move circumferentially in relation to the nose. The jaw assembly is thus free to rotate around the main axis of the nose.
Circumferential operational play is in general provided between two adjacent jaws for making possible a free individual movement of each jaw. This makes it possible in particular to ensure that the insertion operation takes place under the best conditions.
Nevertheless, it happens that all of the jaws wind up crammed together circumferentially on one side of the nose. Thus, a single circumferential operational play is expanded while the other circumferential operational plays are reduced to zero. The result is that the jaws lose certain degrees of freedom of movement, thus creating the danger that the nose will be prevented from being correctly inserted into the neck of the preform.
In addition, the single existing circumferential play accumulates the operational plays that are normally provided. The ring made of elastomer material then runs the risk of forming a hernia in the single circumferential play, locking the jaws in their crammed-together position. In addition, in this configuration, the elastic ring is no longer able to become deformed enough to allow a return of the jaws toward a retracted position. This therefore runs the risk of compromising the ejection operation.